[SuperEditor][Android] Fix scrolling when dragging at empty space (Resolves #2001)

[angelosilvestre]

[SuperEditor][Android] Fix scrolling when dragging at empty space. Resolves #2001

When the user drags starting from an empty space, for example, the area of the document padding, the following crash happens and the editor does not scroll:

════════ Exception caught by gesture ═══════════════════════════════════════════
The following _TypeError was thrown while handling a gesture:
Null check operator used on a null value

When the exception was thrown, this was the stack:
#0      _AndroidDocumentTouchInteractorState._onPanStart (package:super_editor/src/default_editor/document_gestures_touch_android.dart:997:100)
document_gestures_touch_android.dart:997
#1      DragGestureRecognizer._checkStart.<anonymous closure> (package:flutter/src/gestures/monodrag.dart:785:53)
monodrag.dart:785
#2      GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:344:24)
recognizer.dart:344
#3      DragGestureRecognizer._checkStart (package:flutter/src/gestures/monodrag.dart:785:7)
monodrag.dart:785
#4      DragGestureRecognizer._checkDrag (package:flutter/src/gestures/monodrag.dart:752:5)
monodrag.dart:752
#5      DragGestureRecognizer.acceptGesture (package:flutter/src/gestures/monodrag.dart:679:7)
monodrag.dart:679
#6      GestureArenaManager._resolveInFavorOf (package:flutter/src/gestures/arena.dart:282:12)
arena.dart:282
#7      GestureArenaManager._resolve (package:flutter/src/gestures/arena.dart:233:11)
arena.dart:233

The root cause is that, when we check if the user tapped over the caret, we assume that the tap down position can always be mapped to a DocumentPosition, which isn't the case. This only happens if the editor has a collapsed selection.

This PR fixes the issue and adds test for scenarios with collapsed selection, expanded selection and no selection.

[matthew-carroll]

LGTM